The present invention related generally to current measurement probes and more particularly to a current sensing circuit for use in a current measurement probe.
Current probes generally measure current flow in a conductor by sensing the magnetic flux generated by the flow of current in the conductor using a current sensing circuit. The current sensing circuit converts the sensed current signal to a voltage output signal that is coupled to a measurement test instrument, such as an oscilloscope or the like, for display and analysis. The current sensing circuit generally has a transformer input with the transformer having a ring-shaped core of magnetic material. The primary winding of the transformer is the conductor in which the current is to be measured and is disposed within the ring-shaped magnetic core. The current in the primary winding induces a magnetic flux in the magnetic core. The secondary winding of the transformer is wrapped around the magnetic core and is coupled to a termination resistor. The alternating current flowing in the primary winding of the transformer induces a voltage in the secondary winding that produces an alternating current in a direction causing a magnetomotive force or flux in a direction opposing the input flux. The alternating current in the secondary winding is coupled to the termination resistor that converts the alternating current signal to a voltage signal.
Since transformers are AC signal coupling devices, the passband of the transformer cut-off frequency is above the DC level. To allow the current sensing circuit to sense DC and low frequency current signals, a Hall effect device has been included in the magnetic core of the transformer. The Hall effect device is a semi-conductor positioned in the magnetic core such that the magnetic flux in the magnetic core is substantially perpendicular to the Hall effect device. A bias is applied to the Hall plate and the resulting voltage generated by the Hall effect due to the flux in the magnetic core is coupled to the input of a differential amplifier. The single ended output of the amplifier may be coupled to a power amplifier which generates a current output proportional to the voltage generated by the Hall effect device. The output of the Hall device amplifier or alternately the power amplifier is coupled to the secondary winding of the transformer such that the output current from the amplifier flowing through the secondary winding produces a flux that opposes the input magnetic flux over the frequency passband of the Hall effect device. In one implementation, the output of the amplifier is coupled to one side of the secondary winding with the other side of the winding coupled to the transformer termination resistor and amplifier circuitry. In another implementation, the output of the amplifier is coupled via a resistor to the same side of the secondary as the amplifier circuitry. A capacitor is coupled to the input of a wideband amplifier in the amplifier circuitry for blocking the current from the Hall effect amplifier. The output of the Hall effect amplifier and the output of the wideband amplifier are summed at the input of a operational amplifier having a feedback resistor that provides a voltage output proportional to the combined current in the secondary winding of the transformer. The voltage output of the operational amplifier is a measure of the AC and DC components of the magnetic core flux. U.S. Pat. Nos. 3,525,041, 5,477,135 and 5,493,211 describe the above current sensing circuits.
The sensitivity of the above described current sensing circuits are limited by the turns ratio of the transformer and the Hall effect device. As the number of turns in the secondary winding increases relative to the primary winding, the sensitivity of the overall current sensing circuit decreases. In addition, the inclusion of the transformer termination resistor or the use of a resistor for coupling the output the Hall effect amplifier to the secondary winding of the transformer causes the low frequency cut-off point of the transformer frequency response to increase as a function of L/R where L is the inductance of the secondary winding and R is the resistance of the secondary winding and the transformer termination resistor. This requires that the high frequency cut-off point of the combined Hall effect device and amplifier frequency response to extend to past the low frequency cut-off point of the transformer frequency response for a smooth crossover from DC to low frequency response of the Hall effect device to the high frequency response of the transformer. Further, the prior art designs requires that the Hall effect amplifier provide sufficient current to null the DC to low frequency flux generated in the magnetic core of the transformer. The Hall effect device amplifier provides a current output that generally requires the use of a power amplifier for current probing devices having a maximum current ratings in the tens of amps range.
The '135 and '211 patents suggest that the voltage amplifier circuitry having the transformer termination resistor may be replace with a transimpedance amplifier since the transimpedance amplifier may be used instead to develop a voltage output in response to a current input signal. However, such a current sensing circuit would still require the use of the Hall effect amplifier and power amplifier to generate a bucking current that is applied to the secondary winding of the transformer.
What is needed is a current sensing circuit where the Hall effect amplifier does not generate the current signal that is applied to the secondary winding of the transformer. Further, there is a need for a current sensing circuit that does require the use of a power amplifier for generating the bucking current to null the DC to low frequency flux in the magnetic core of the transformer. Such a circuit should have a high current to voltage gain with low input resistance which increases the sensitivity of the current sensing circuit.